Pacific Gas & Electric wants to close five older, higher-polluting power plants in East Contra Costa as part of a statewide public utilities drive to make the power grid cleaner.

Two of the plants are in Pittsburg, two are in Antioch and one is in Bay Point. The plants are, on average, about 20 years old and operate by burning petroleum coke, a byproduct of oil refining.

The fate of nearly 100 workers has not been determined, although some may be offered jobs at a Tracy plant.

PG&E has asked the state’s Public Utilities Commission for permission to close the plants, which are operated by Pittsburg-based GWF Energy LLC.

The plants together produce about 100 megawatts of power that is sold to PG&E for the statewide power grid. A megawatt can power 1,000 homes when produced constantly.

Closing the plants will reduce greenhouse gas emissions by 600,000 tons a year and save ratepayers $15 million over the 10 years remaining on a 20-year contract between GWF and PG&E because the plants’ output will be replaced by energy that is cheaper to produce, PG&E spokesman Denny Boyles said.

GWF is converting a cleaner-burning natural gas-fired plant in Tracy to increase its output by 144 megawatts to 314 megawatts.

“The Tracy plant will replace the energy that’s being taken offline from the East County plants and more,” said Vicki DiBernardo, GWF’s human resources director.

While it takes out older plants, PG&E also is looking to invest in renewable sources of energy, Boyles said.

PG&E has begun operating or purchasing power from three new gas-fired power plants in East Contra Costa over the past 10 years, and at least three more are in the works.

An environmental activist who follows the power industry said he supports retiring the coke-fired plants but still thinks that PG&E has more than enough fossil-fuel capacity in the Bay Area.

“PG&E needs to get rid of these old stinkers and nuclear power,” said Michael Boyd, president of Californians For Renewable Energy, which is based in Soquel. “We can increase capacity by requiring PG&E to buy solar and other renewable energy from small producers, not just from plants with 20 megawatts and above.”

Investor-owned utilities are using as much as 20 percent renewables and want to get to 33 percent by 2020, said Michael Kanellos, editor of Greentech Media in San Francisco.

GWF has about 90 workers at the East County plants and its Pittsburg headquarters, DiBernardo said.

Some East County workers may be offered work in Tracy if the closures go through as anticipated at the end of January, she said.

“It hasn’t been determined yet how many of the (East County) positions will be eliminated,” DiBernardo said.

The state’s investor-owned utilities, such as PG&E and Southern California Edison, are moving to natural gas as an alternative to fuels such as petroleum coke that are similar to coal, Kanellos said.

Gas will account for close to 40 percent of electricity produced nationwide by 2035, up from 20 percent today, he said.

“You get about half the CO2 compared to coal, and you have a lot less particulate matter,” Kanellos said. “Investors are very worried about carbon regulations and don’t want to have old, smelly power plants on the books.”

Coke-fueled plants produce synthetic gypsum as a byproduct used in manufacturing cement.

Refineries often power their own operations with the petroleum coke they don’t sell to power plant operators.

The closure application to the Public Utilities Commission also includes a request to extend contracts to operate two GWF natural gas peak-demand plants in the Central Valley, according to the company.

Crews from several fire departments are battling a major grass fire late Saturday afternoon that has claimed at least 500 acres in a rural area in Solano County between Vacaville and Winters, and is prompting mandatory evacuations, firefighters said.